1. Field of the Invention
The present invention relates to a method for producing polarizing films for use in liquid-crystal displays.
2. Description of the Related Art
A polarizer having the function of transmitting and blocking out light is a basic constitutive element in liquid-crystal displays (LCD), along with the liquid crystal that functions as a switch for light therein. Regarding their application field, LCDs were almost for small-sized instruments such as electronic calculators and wristwatches in the early days at the beginning of their development, but in these days, their applications are much expanding for other various instruments such as notebook-size personal computers, liquid-crystal monitors, liquid-crystal color projectors, liquid-crystal televisions, navigation systems for automobiles, personal phones, as well as measuring instruments for indoor and outdoor use, etc. Especially in liquid-crystal monitors and liquid-crystal televisions in the field of such LCD applications, bright backlights are often used, for which the polarizers are required to have better polarizing properties than those of conventional ones.
In general, polarizers are constructed by laminating a polarizing film with a protective film such as a triacetyl cellulose (TAC) film or a cellulose acetate butyrate (CAB) film, for which the polarizing film is prepared by monoaxially stretching and coloring a polyvinyl alcohol film (polyvinyl alcohol will be hereinafter abbreviated as “PVA”; and the polyvinyl alcohol film will be as “PVA film”) with iodine or dichroic dye, or coloring and then monoaxially stretching it, followed by treating it with a boron compound for fixation (as the case may be, coloration and fixation may be effected at the same time).
With the development of large-sized screens in liquid-crystal monitors and liquid-crystal televisions, now required are polarizing films that are wider than conventional ones. For producing such wide polarizing films, wide PVA films enough for them must be used. However, if wide PVA films are produced by the use of the same production apparatus and under the same production condition as those for conventional PVA films, it is problematic in that the polarizing properties of the wide PVA films produced in that manner are poorer than those of ordinary PVA films.
On the other hand, liquid-crystal monitors and liquid-crystal televisions are required to enjoy more improved contrast. To meet the requirement, desired are polarizing films that have better polarizing properties than those of conventional ones. As so mentioned hereinabove, however, if wide polarizing films enough for such large-sized screens in liquid-crystal monitors and liquid-crystal televisions are produced, the polarizing properties of the wide polarizing films thus produced are not good. This means that the wide polarizing films could not meet the requirement of contrast improvement in liquid-crystal monitors and liquid-crystal televisions.
For one trial of improving the polarizing properties of polarizing films, proposed is a process of controlling the stretching condition in monoaxially stretching PVA films. For it, for example, known are a method of preparing a PVA film through casting or extrusion of a mixture of PVA, a dichroic substance and a solvent, followed by stretching it under dry heat and under a controlled condition of a ratio of the non-stretched film width (c0) to the stretching gap (l), c0/l of at most 3 (JP-A 6-51123 and U.S. Pat. No. 5,326,507); a method of stretching a PVA film in an aqueous boric acid solution under a controlled condition of a ratio of the stretching distance (a1) to the film width (c1), a1/c1 of at least 3 (JP-A 6-337311); and a method of stretching a PVA film in an aqueous boric acid solution on such a controlled condition that the film width of the stretched film is at most 60% of that of the non-stretched film (JP-A 8-327823).
Even in these methods, however, wide PVA films could not be processed into polarizing films having good polarizing properties. At present, therefore, polarizing films known in the art are not enough for large-sized liquid-crystal displays.
An object of the invention is to provide a method for producing polarizing films having good polarizing properties by monoaxially stretching wide polyvinyl alcohol films.